In a transport network system, a signal flow needs to be “dropped” from a node, or a signal flow needs to be “added” to a node. A device that “drops” and “adds” a signal flow is referred to as an add/drop multiplexer (ADM). The ADM includes an optical add/drop multiplexer (OADM) and an electrical add/drop multiplexer (EADM).
Due to exchangeability of communication information, a transport network is generally bidirectional, as shown in FIG. 1. A service P26 between a node N2 and a node N6 is bidirectional communication information, is added and dropped by using ADMs of the nodes, and passes through an ADM of a node N1. Similarly, a service P36 between a node N3 and the node N6 is bidirectional communication information, is added and dropped by using ADMs of the nodes, and passes through ADMs of a node N4 and a node N5. As shown in FIG. 2, an ADM generally includes a line board (Line Card) 201, a cross-connect board (Switch Card) 202, a line board 203, and a tributary board (Tribute Card) 204. The boards are independent boards, which is convenient for maintenance. When a board is faulty, communication on an entire link may not be affected. The boards are connected to each other through an inter-board interface. A signal on an eastbound link is input into the line board 201, is processed and then sent in a downlink direction to the cross-connect board 202 through an inter-board interface (1), is sent in the downlink direction to the line board 203 through an inter-board interface (2), and is processed and then output to an egress of the eastbound link. A signal on a westbound link is input into the line board 203, is processed and then sent in a downlink direction to the cross-connect board 202 through the inter-board interface (2), is sent in the downlink direction to the line board 201 through the inter-board interface (1), and is processed and then output to an egress of the westbound link. After the cross-connect board 202 receives the input signal on the eastbound link from the line board 201 through the inter-board interface (1), or receives the input signal on the westbound link from the line board 203 through the inter-board interface (2), a downlink service of a current node is extracted from a signal flow on the eastbound link and/or a signal flow on the westbound link, is processed by using a combiner/selector and then sent to the tributary board 204 through an inter-board interface (3), and is transferred to a device on a client side. The cross-connect board 202 receives an uplink service of the current node from the tributary board 204 through the inter-board interface (3). The uplink service of the current node is processed by using a replicator/distributor and then inserted into the signal flow on the eastbound link and/or the signal flow on the westbound link, and enters the line board 203 through the inter-board interface (2), or enters the line board 201 through the inter-board interface (1), to form an output signal on the eastbound/westbound link.
Because there is an inter-board interface between the line board and the cross-connect board of the ADM, and the eastbound/westbound link passes through the inter-board interface between the line board and the cross-connect board, a link bandwidth processing capability of the ADM is limited to bandwidth of the inter-board interface between the line board and the cross-connect board, and a maximum of the link bandwidth processing capability of the ADM does not exceed the bandwidth of the inter-board interface between the line board and the cross-connect board.